1. Field of the Invention
This invention relates to optical receivers, and more particularly, for conversion of optical signals into electrical signals.
2. Description of the Related Art
Optical systems are commonly used for the transmission of data signals. The high speed of optical signals allows for high information bandwidth. Thus, systems for transmitting and receiving optical signals can be found in many applications, such as computer networks and communications systems.
Information transmitted using optical signals is originated as information carried by electrical signals. These electrical signals are converted to optical signals and conveyed by an optical transmitter. The optical signals may then be received by an optical receiver and converted back into electrical signals for use in a system that recovers the transmitted information. This conversion of signals from electrical to optical and back to electrical again may introduce bottlenecks and other problems into systems based on optical signaling.
FIG. 1 is a block diagram of a portion of one embodiment of an optical receiver. In the embodiment shown, an optical signal may be received by a photodiode. The optical signal induces an electrical current through the photodiode, which may be received by the high gain amplifier path and the low gain amplifier path. Each of the high gain and low gain paths is coupled to a filter, which removes unwanted noise from its respective path. A switch couples one of the high gain or low gain paths to an analog-to-digital converter (ADC). The control unit is configured to monitor the signal levels on both signal paths and operate the switch to couple either the high gain or low gain signal path to the ADC. More particularly, the control unit is configured to couple the high gain path or the low gain path to the ADC depending on the signal amplitude, wherein the high gain path is used to amplify weak signals and the low gain path is used to amplify stronger signals. The control unit may operate the switch to couple the high gain path to the ADC if the amplitude of the output signal from the low gain path falls below a certain level, or couple the low gain path to the ADC if its amplitude exceeds this level.
Other types of receivers include a single amplifier with multiple feedback paths (wherein the switch selects a feedback path based on a signal level) or those that include a single filter, with the switch coupling either a high gain or low gain path to the filter.
These types of receiver may be subject to various problems. For example, when switching between the high gain and low gain paths in the embodiment of FIG. 1, the ADC may require settling time before a valid output can be produced. In an embodiment utilizing a single filter, switching may require settling time for the filter before a valid output is produced. In embodiments utilizing a single amplifier, switching may necessitate a settling time for the amplifier to produce a valid output.
Thus, in addition to any time lag induced by the control circuit in determining when to switch from one path to the other, settling times from amplifiers, filters, and/or an ADC may result in invalid data and can thus limit the speed at which the receiver may receive and accurately convert received optical signals into digital data signals. Another problem includes the possible presence of DC bias voltages on the photodiode, which can affect the accuracy of the data. Thus, the various problems of the optical receiver circuit shown in FIG. 1 and other embodiments described above may prevent it from being used in applications where data must be received and accurately converted into digital signals at very high speeds.